A large number of image formation methods using a photopolymerization system have been conventionally known and widely used in the fields such as printing plate, printed circuit, paint, ink, hologram recording and three dimensional formation. For example, there are known a method where a photopolymerizable composition comprising an addition-polymerizable compound having an ethylenic double bond, a photopolymerization initiator and further, if desired, an organic polymer binder and a thermal polymerization inhibitor, is provided on a support to form a film layer, the layer is imagewise exposed to a desired image to cure the exposed area by polymerization and then the unexposed area is dissolved and removed to form a cured relief image, a method where a layer comprising the above-described photopolymerizable composition is provided between two supports, at least one support being transparent, the layer is imagewise exposed from the transparent support side to induce change in the adhesive strength by light and then the support is peeled off to form an image, a method where a photosensitive material having a microcapsule layer containing a photopolymerizable composition and a coloring material such as a leuco dye is prepared, the photosensitive material is imagewise exposed to photocure the capsules in the exposed area, the capsules in the unexposed area are ruptured by pressure or heat treatment to put it into contact with the coloring material developer to effect color formation to thereby form a color image, an image formation method using the change in the toner adhesion property of the photopolymerizable composition due to light, and an image formation method using the change in the refractive index of the photopolymerizable composition due to light.
The photopolymerizable composition applied to these methods uses in many cases benzyl, benzoin ether, Michler's ketone, anthraquinone, acridine, phenazine or benzophenone as a photopolymerization initiator. However, these photopolymerization initiators are extremely low in the photopolymerization initiation ability to the visible light of 400 nm or more as compared with the photopolymerization initiation ability to the ultraviolet light of 400 nm or less and accordingly, these are remarkably limited in the application range.
Recently, as the image formation technique has been developed, a photopolymer having high sensitivity to light rays in the visible region is being demanded. The photopolymer is a photosensitive material suitable, for example, for non-contact type plates using projection exposure or for plates using a visible light laser. Highly expected as the visible light laser are Ar.sup.+ laser which emits light of 488 nm and a YAG-SHG laser which emits light of 532 nm.
With respect to the photopolymerization initiation system sensitive to light rays in the visible light region, many proposals have been hitherto made. Examples of the system include a certain kind of sensitive dyes described in U.S. Pat. No. 2,850,445, a composite initiation system of a dye and an amine (see, JP-B-44-20189 (the term "JP-B" as used herein means an "examined Japanese patent publication")), a combination system of hexaarylbiimidazole, a radical generating agent and a dye (see, JP-B-45-37377), a system of hexaarylbiimidazole and p-dialkylaminobenzylidene ketone (see, JP-B-47-2528, JP-A-54-155292 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), a system of a cyclic cis-.alpha.-dicarbonyl compound and a dye (see, JP-A-48-84183), a system of a substituted triazine and a merocyanine dye (see, JP-A-54-151024), a system of 3-ketocoumarin and an activator (see, JP-A-52-112681, JP-A-58-15503), a system of biimidazole, a styrene derivative and a thiol (see, JP-A-59-140203), a system of an organic peroxide and a dye (see, JP-A-59-140203, JP-A-59-189340) and a system of a dye having a rhodanine skeleton and a radical generating agent (see, JP-A-2-244050).
The titanocene is effective as a photopolymerization initiator as described in JP-A-59-152396, JP-A-61-151197, JP-A-63-10602, JP-A-63-41484 and JP-A-3-12403, and examples of the combination use system thereof include a system of titanocene and a 3-ketocoumarin dye (see, JP-A-63-221110), a system where titanocene, a xanthene dye and further, an addition-polymerizable ethylenically unsaturated compound containing an amino group or a urethane group are used in combination (see, JP-A-4-221958, JP-A-4-219756) and a system of titanocene and a specific merocyanine dye (see, JP-A-6-295061).
These conventional techniques are surely effective to visible light rays, however, they are bound to problems such that the sensitivity is insufficient, hence, these systems cannot be used in practice.